|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|8. Three-phase Current|
In the preceding Chapter we have shown that a sinusoidal alternating voltage is induced in a conductor loop which is rotated in a homogeneous magnetic field. Several conductor loops which are mechanically connected together can also be turned in a magnetic field at the same time. Then, in each conductor loop, a sinusoidal alternating voltage is induced. An arrangement where three conductor loops displaced by 120 to each other are used has gained great importance in practice (Fig. 8.1.).
Fig. 8.1. Principle of the three-phase oberhung-type alternator
1, 2, 3 - Rotatable coils
4 - North pole
5 - South pole
In order to conduct the electrical energy generated in this overhung-type alternator to the consumer, sliding contacts must be provided for all three conductor loops. This disadvantage is not associated with the inner-pole alternator. In This type, the three conductor loops are fixed in the stator of the alternator (an alternating-current generator is also called alternator) while the magnetic field in the interior is rotated. (Fig. 8.2.). The relatively low electrical energy for the production of the magnetic field must be fed to the rotor of the generator via sliding contacts.
In each of the three coils, a sinusoidal voltage is produced when the magnetic field is rotating which - in accordance with the arrangement of the coils - exhibits a phase shift of 120°. (Consequently, a voltage maximum is always reached, when the magnetic pole is turned past the coil. For the three coils, this always occurs after a rotation of the magnetic pole through 120 °). The line diagram of these three voltages is shown in Fig. 8.5.
Fig. 8.2. Principle of the three-phase inner-pole alternator
1, 2, 3 - Fixed coils
4 - North pole
5 - South pole
Fig. 8.3. Line diagram of voltages of three-phase current
The three voltages subjected to a phase-shift of 120° are called three-phase current. In order to distinguish safely between the three voltages and their three coils, the three phases are marked by L1, L2 and L3 and coloured (L1: yellow; L2: green; L3: violet) according to an IEC-recommendation. 1) The connections of the starts of the coils can be designated by U, V and W and the ends of the coils by X, Y and Z. For the phases L1 to L3, sometimes R, S and T are used as designation.
1) IEC = International Electrotechnical Commission
When a magnetic field is rotated, inside of three coils which are displaced by 120° to each other, then three sinusoidal voltages are produced which are called three-phase current. A phase shift of 120° exists between every two of the three voltages.
1. What is the difference between overhung-type alternator and inner-pole alternator?
2. What are the advantages of the inner-pole alternator over the overhung-type alternator?
3. How are the three phases distinguished from each other by markings?
4. What is the phase angle between the voltages of the three phases?